The Science of E-cigarette Formaldehyde
The point of an e-cig is not, firstly, to get people hooked on nicotine. It was invented for a simple purpose, and that was to create a low temperature vaporiser that could deliver nicotine much more safely to people who were already addicted and seeking a safer alternative. The key was the low temperatures that avoided combustion products which are the primary source of toxins in a tobacco cigarette.
As a result it was always the case that an e-cig was much safer provided the power delivered to the atomiser was not so high that low-temperatures instead became high temperatures, and also it was assumed that the atomiser had e-liquid delivered to it efficiently so that it did not run 'dry'. When the atomiser is heated the e-liquid vaporises, carrying away heat and keeping the maximum temperature low. If it runs dry though, this cooling is lost and the liquid residue on the atomiser coil can get hotter, and then you get the heating conditions that the original e-cigarette was designed to avoid.
Recent studies have been published that prove what was already predicted and known from earlier research sponsored by the e-cig industry - that 1) high power 'second generation' variable-voltage e-cigarettes on high power settings (around 5 volts, equivalent to 12-15 Watts) when also combined with dry puff conditions will cause much higher temperatures (from the 200-260 degrees Celsius in normal e-cigarettes, jumping to a whopping 600 degrees Celsius!), and that 2) these much higher temperatures will definitely produce some free-radicals like an ordinary cigarette does when it burns.
The problem everyone is talking about comes from the combination of variable wattage/voltage batteries when combined with 'top-coil' atomisers. A top-coil atomiser has the the atomiser placed higher up in the tank along with the wick, which means juice easily runs out as it is used up at the atomiser coil. This increases the temperature further, creating combustion products. There is another kind of atomiser called a 'bottom-coil' atomiser, which sits near the base of the tank. As a result the eliquid is flowing constantly and at a greater rate into the atomiser coil, greatly reducing the chance of dry-puff. When these were tested, HARDLY ANY FORMALDEHYDE WAS GENERATED.
The recent study published in the NEJM which has generated a media storm, actually showed that low power devices or bottom coil tanks were not producing any significant level of free-radicals, and much less than cigarettes. It also showed they are not producing other types of toxins associated with burning tobacco such as nitrosamines which are recognised as much more potent carcinogens. These are not produced in e-cigs even at high temps because nitrogen containing chemicals like amino-acids found in tobacco, are not present in e-liquid and these are needed to make the highly carcinogenic tobacco-nitrosamines under certain conditions.
See Spreading fear and confusion with misleading formaldehyde studies for more details about dry-wick phenomena and the temperatures involved in certain types of hi-power, top-coil atomisers
....And see here for a proper analysis of the research paper published in the NEJM, showing that it is certain combinations of e-cig that can produce free-radicals; the top-coil atomiser along with high power settings on a variable voltage battery combined with long puff duration and short periods between puffs. These chemicals would be easily tasted by the user and avoided;
Consumers are extremely unlikely to voluntarily inhale high concentrations of formaldehyde; formaldehyde is characterized by its unpleasant smell, and at concentrations of as little as 5 parts per million causes burning sensations in the respiratory tract, and breathing difficulty